1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases.
2. Description of the Related Art
Exhaust gases from automotive engines and other internal combustion engines contain harmful components such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). If exhaust gases containing these harmful components are emitted into the air without any treatment, pollution and environmental degradation will occur. Therefore, exhaust gases containing these harmful components are emitted into the air after purified by such a purifying apparatus as a catalyst for purifying exhaust gases.
The catalyst for purifying exhaust gases converts NOx, HC and CO contained in exhaust gases into innocuous nitrogen, carbon dioxide or water by catalyst metal.
It is known that HC conversion is strongly influenced by temperature and generally carried out at not less than 300° C. When catalyst temperature is low, catalytic activity of catalyst metal is too low to convert HC in exhaust gases. The catalyst for purifying exhaust gases is heated by exhaust gases. This fact indicates that the catalyst soon after an engine starts has a low temperature. Besides, exhaust gases immediately after an engine starts contain a large amount of HC and the ratio of HC in the emission is large.
Accordingly, it is a critical problem to suppress HC emission from a catalyst when the catalyst has a low temperature.
This catalyst for purifying exhaust gases which suppresses HC emission can be produced, for example, by forming a catalyst carrier layer composed of alumina, etc. on the surface of a catalyst support and forming a HC conversion part for converting HC and a NOx conversion part for converting NOx and CO separately on the catalyst carrier layer. In this catalyst, there are two kinds of separation of the HC conversion part and the NOx conversion part: separation in an axial direction of the catalyst support and separation in a thickness direction of the catalyst carrier.
Examples of this catalyst are disclosed in Japanese Unexamined Patent Publication Nos. H11-253758, H11-104462, H7-213910, H6-142519, H11-210451, and H11-221466.
Japanese Unexamined Patent Publication No. H6-142519 discloses a hydrocarbon adsorptive catalyst which is provided with a first layer of ZSM-5 zeolite ion-exchanged with at least one metal of Cu and Pd, which are effective for hydrocarbon adsorption, on a monolithic carrier, a second layer of a powder based on activated ceria and/or alumina containing at least one metal of Pt and Pd as a catalytic component on the first layer, and a third layer containing Rh as a catalytic component on the second layer.
Japanese Unexamined Patent Publication No. H7-213910 discloses an exhaust gas purifying catalyst comprising an adsorbing catalyst formed of a catalyst carrier and a zeolite layer coated thereon, wherein a catalyst layer composed of a powder based on activated ceria and/or alumina containing at least one element of Pt, Pd and Rh as a catalytic component is provided on the zeolite layer.
Japanese Unexamined Patent Publication No. H11-104462 discloses a catalyst-adsorber for purifying exhaust gases, which is prepared by covering and loading an adsorbing layer composed of an adsorbent with hydrocarbon adsorbability on a monolithic carrier and covering and loading a catalyst layer composed of a catalyst material with purifying ability for harmful components in exhaust gases on the adsorbing layer, the catalyst layer having a thickness of 10 to 120 μm.
Japanese Unexamined Patent Publication No. H11-210451 discloses an exhaust gas purifying catalyst device formed by providing, in exhaust passages, a HC-adsorbing catalyst comprising a double-layered part constituted by a HC-adsorbent lower layer and a three-way catalyst upper layer, and a single-layered part constituted by a three-way catalyst layer alone and positioned on the downstream side of the exhaust emission flow.
Japanese Unexamined Patent Publication No. H11-221466 discloses a catalyst for purifying exhaust gases, wherein a catalyst containing zeolite is arranged in a front stage part to an exhaust gas flow, and a catalyst consisting of three catalyst layers is arranged in a rear stage part to the exhaust gas flow. Of the three catalyst layers, the first catalyst layer contains at least one component of platinum (Pt), palladium (Pd) and rhodium (Rh) and at least one component of alkali metals, alkaline earth metals, and rare earth elements, the second catalyst layer contains alumina and/or silica, and the third catalyst layer contains zeolite including a copper component and a cobalt component, and the first, second and third catalyst layers are overlaid in turn.
Japanese Unexamined Patent Publication No. H11-253758 discloses a catalyst for purifying exhaust gases comprising: a first catalytic layer containing one or more components of a platinum component, a palladium component and a rhodium component and one or more kinds of components of alkali metals, alkaline earth metals and rare earth elements; a second catalytic layer containing β-zeolite, formed on the first catalytic layer; and a third catalytic layer which is composed mainly of zeolite containing a copper component and/or a cobalt component and formed on the second catalytic layer. In this case, the second catalytic layer structurally contains silica (SiO2).
Moreover, as evidenced by severer automotive emission limits, there is a demand for an improvement in HC purifying ability and exhaust gas purifying ability of a catalyst for purifying exhaust gases.